The material of a lighting fixture body and a lens forming a conventional vehicle lighting fixture has been changed from metal or glass to polycarbonate or acrylic resin. The polycarbonate, acrylic resin or other plastic material is superior in impact strength, thermal stability, electric characteristics, antiweatherability and dimensional stability.
The hot melt adhesive having thermal plasticity is the agent mainly used for bonding the body and the lens of the lighting fixture, because it has a high adhesion and provides high productivity.
In the conventional method, the hot melt adhesive is heated and melted at between about 190.degree. C. and about 200.degree. C., and applied using a hot melt adhesive applicator to the sealing grooves integrally formed in the molded polypropylene resin lighting fixture body. Subsequently, the anchoring parts of the lens are press-fit into hot melt adhesive in the sealing grooves, and fixed mechanically by using spring pegs or other.
When the polycarbonate lighting fixture body and lens are thus bonded together and heated to between 60.degree. C. and 120.degree. C., however, water molecules, carbon dioxide and other low molecular weight compounds are generated from polycarbonate and reside as foam in the interface between polycarbonate member and hot melt adhesive, thereby deteriorating the bonding or sealing properties.
The similar phenomenon is found when the acrylic resin lighting fixture body and lens are bonded together.
To solve such problem, polycarbonate resin is heated and dried so as to remove hydrogen oxide, carbon dioxide and other low molecular weight compounds in advance before it is applied in assembling the lighting fixture body and the lens. For this purpose, the heating apparatus and process are required.
.Japanese Laid-open Patent Application No. 3-21682 discloses another solution. In the prior art, after the hot melt adhesive is melted, during a series of process for applying the hot melt adhesive to the plastic, pressure is reduced by no less than 500 mmHg. At the additional step of degassing the hot melt adhesive, the residual foam is eliminated from the interface between the plastic and the hot melt adhesive. Therefore, the prior art requires the additional step of pressure reduction and degassing, and the associated apparatus.